In recent years, batteries as power sources of mobile devices and the like have been strongly required to have light weight designs and thin designs. Therefore, also for covering materials for batteries, those using metal thin films or laminated films obtained by laminating a metal thin film and a thermally-fusible resin film have come to be used in place of conventional metal cans having limits in terms of light weight designs and thin designs, because laminated films are capable of accommodating lighter weight designs, and thinner designs and accommodating free shapes compared to metal cans.
Representative examples of a laminated film used as a covering material for a battery include a three-layer laminated film in which a thermally-fusible resin film as a near shield layer is laminated on one surface of an aluminum thin film, which is a metal thin film, and a protective film is laminated on the other surface.
In a film-covered battery using a laminated film as a covering material, a battery element in which a positive electrode and a negative electrode are stacked via a separator is enclosed with a laminated film, with thermally-fusible region films opposed to each other, and the laminated film is thermally fused along the circumference of the battery element, whereby the battery element is hermetically sealed (hereinafter hermetical sealing is simply called sealing). The battery element has a collector portion formed by collectively bonding each of the positive electrode plates and negative electrode plates, which extend outward from a stacked region where a plurality of positive electrode plates and a plurality of negative electrode plates are stacked in an opposed condition.
In order to draw out the positive electrode and negative electrode of the battery element to the outside of the laminated film, a tab is connected to a terminal part of the collector portion of the positive electrode and the negative electrode. These tabs are such that a part thereof extends outward from the laminated film.
The battery element is such that the positive electrode and the negative electrode are stacked via a separator. It is necessary to ensure that the positive electrode and the negative electrode do not come apart from each other during the manufacturing process. If, after completing assembly of a battery, the positive electrode, the negative electrode and the separator become displaced from each other in the interior of the battery, then it becomes impossible to obtain the prescribed characteristics.
Similar problems occur also in a wound-type battery element. In the case of a battery element wound in a spiral form, the loosening of a winding stop makes the distance between electrodes nonuniform and induces the deterioration of battery performance. Therefore, in JP10-241744, fixing means for a winding stop is disposed on the side surface of an electrode body parallel to the winding axis of the electrode body, on a top surface of the electrode body or/and a bottom surface of the electrode body. This fixing means has pores. The pores are provided so as not to prevent the movement of an electrolyte and gas.
On the other hand, also in a stacked-type battery element, a battery using a film which wraps a battery element is disclosed in JP11-265693. The battery disclosed in JP11-265693 uses a solid electrolyte. This solid electrolyte pollutes a sealing portion of the laminated film in the manufacturing stage and, therefore, the reliability of the sealing portion becomes very low. Therefore, in order to prevent this, the battery element is wrapped with a synthetic resin film. For this reason, no pore can be formed in the synthetic resin film disclosed in JP11-265693. If pores are formed, solid electrolyte leaks from the pores and pollutes the sealing portion, with the result that the effect of wrapping with a film cannot be obtained.
In addition, in order to prevent a stacked-type battery element from coming apart, as shown in FIG. 1A, a method which involves fixing with fixing tape is sometimes adopted. In FIG. 1A, areas in the vicinity of four corners of a battery element are fixed with four pieces of fixing tape.
In the manufacturing of a stacked-type film-covered battery, it is necessary to ensure that the battery element does not come apart, and as well, the insulating properties of a collector portion and tabs pose a problem.
Fixing means 10 of JP10-241744 is provided between lead terminals 5, 5′ and hence it is impossible to achieve insulation between areas in the vicinity of lead terminals 5, 5′ as well as corners of the electrode body not covered with fixing means 10 and a battery container by using fixing means 10.
It is stated that it is preferred that polyester film 5 of JP11-265693 be completely wrapped with a synthetic resin film on its at least four surfaces except the surface where a terminal portion is present, preferably on its five surfaces, and for the length of the film, it is stated that this length is 150 to 300% of the length of a peripheral edge portion of the battery element. In JP11-265693, it is stated that wrapping with a synthetic resin film includes wrapping with a synthetic resin film having a larger width than the battery element or folding back a protruding synthetic resin film inside. However, it can be said that in the film of JP11-265693, no consideration is given to overlaying with a synthetic resin film for the surface where a terminal portion is present. That is, it can be said that in the film of JP11-265693, no consideration is given to the insulating properties in the terminal portion.
The present inventors have developed a sack-shaped member for improving the insulating properties between a collector portion or a tab and a covering film. By covering the collector portion and the tab with the sack-shaped member, it is possible to easily achieve the improvement of the insulating properties of these parts (pamphlet of International Publication No. 2005-086258).